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Air pollution

Types of air pollution

The most common and harmful pollutants outdoors include:

We also have information on indoor air pollution.

Particulate matter (PM)

What is particulate matter (PM)?

Particulate matter is a mix of solids and liquids, including carbon, complex organic chemicals, sulphates, nitrates, mineral dust, and water suspended in the air.

PM varies in size. Some particles, such as dust, soot, dirt or smoke are large or dark enough to be seen with the naked eye. But the most damaging particles are the smaller particles, known as PM10 and PM2.5. PM10 refers to particles with a diameter smaller than 10 microns (10µm) – that’s 100 times smaller than a millimetre. PM2.5 refers to particles with a diameter smaller than 2.5 microns, and these are known as fine particles. The smallest fine particles, less than 0.1 micron in diameter, are called ultrafine particles.

Diameters of particulate matter compared to a human hair

Where does PM come from?

Man-made particulate matter mainly comes from industry, building work, diesel and petrol engines, friction from brakes and tyres, and dust from road surfaces. Diesel engines tend to produce much more than equivalent petrol engines.

Natural sources of particulate matter include volcanoes, sea spray, pollen and soil. It is also formed in the atmosphere when gases such as nitrogen dioxide and sulphur dioxide are changed in the air by chemical reactions.

How does PM affect your lungs?

The size of particulate matter will determine where it will end up once you breathe it in. Larger particles may be trapped in your nose, while PM10 can reach your airways. Fine particles (PM2.5) may reach the breathing sacs deep in your lungs, and ultrafine particles may even cross into your blood stream. These particles can also carry toxic chemicals that are linked to cancer.

Particulate matter irritates your nose and throat and may be associated with more severe symptoms in people with asthma. It results in more people with lung conditions (COPD, asthma, bronchitis) and heart conditions (heart attacks, strokes) being admitted to hospital. It also causes early deaths from lung and heart disease.

There’s also evidence that long-term exposure to particulate matter can contribute to the development of lung cancer and possibly asthma.

Nitrogen dioxide (NO2)

What is nitrogen dioxide (NO2)?

Nitrogen dioxide is a gas and is a major component of urban air pollution episodes.

Where does NO2 come from?

Man-made sources of nitrogen oxides, including nitrogen dioxide, are vehicles, power stations and heating. Diesel vehicles are major contributors in urban areas. Roadside levels are highest where traffic is busiest.

How does NO2 affect your lungs?

High levels of NO2 can irritate and inflame the lining of your airways, causing a flare-up of asthma or COPD and symptoms such as coughing and difficulty breathing.

Children and older people are also more likely to be affected and develop a respiratory infection and may react more to allergens (any substance that triggers an allergic reaction, such as pollen).

Ozone (O3)

What is ozone?

Ozone is a gas composed of 3 atoms of oxygen. In the upper level of the Earth’s atmosphere, it absorbs harmful ultraviolet radiation.

Where does ozone come from?

Near the ground, ozone is made by a chemical reaction between the sun’s rays and organic gases and oxides of nitrogen emitted by cars, power plants, chemical plants and other sources.

Ozone is usually highest in the spring and summer and lowest in the winter. Ozone levels are highest during the afternoon and are often higher in the country than in towns. Ozone is a major component of summer air pollution episodes.

How does ozone affect your lungs?

Ozone can irritate the airways of both healthy people and those with lung conditions. High levels can cause discomfort when you breathe, reduce your lung capacity (the amount of air your lungs can hold) and trigger asthma symptoms.

If you have a lung condition, high levels of ozone can cause difficulty breathing, wheezing and coughing. People with asthma may need to use their reliever inhaler more often.

When there are high levels of ozone, more people are admitted to hospital with asthma-related health problems and COPD symptoms, and there is a greater risk of illnesses like pneumonia and bronchitis.

Sulphur dioxide (SO2)

What is sulphur dioxide?

Sulphur dioxide is a colourless gas, with a pungent, suffocating smell. It’s produced by burning sulphur-containing fuels such as coal and oil. This includes vehicles, power generation and heating.

Where does sulphur dioxide come from?

Most sulphur dioxide comes from electric industries that burn fossil fuels, and also from petrol refineries and cement manufacturing. It can travel over long distances and contributes to the formation of ozone.

How does sulphur dioxide affect your lungs?

Sulphur dioxide can irritate the lining of your nose, throat and lungs. It can cause coughing and tightness of your chest, as well as a narrowing of your airway that will reduce the flow of air to your lungs. It inflames the airways, causing coughing and more mucus. It makes conditions like asthma and COPD worse and can lead to people being more prone to chest infections.

People with asthma are much more sensitive to sulphur dioxide than those who do not have asthma. They may find breathing more difficult and have flare-ups when concentrations of sulphur dioxide are high.

Next: The effects of air pollution >

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Download our air pollution information (150KB, PDF)

Last medically reviewed: April 2020. Due for review: April 2023

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.